Switching device compensation circuit

ABSTRACT

A switching device compensation circuit performs switching control by applying a control pulse to a control terminal of a switching device. The switching device compensation circuit includes a first threshold voltage change detection unit, a first control signal generating unit, and an amplitude control unit. The first threshold voltage change detection unit detects a change in threshold voltage of the switching device from an output voltage controlled via the switching device. The first control signal generating unit generates a first control signal in accordance with an output of the first threshold voltage change detection unit. The amplitude control unit controls the amplitude of the control pulse in accordance with an output of the first control signal generating unit.

CROSS REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority of theprior Japanese Patent Application No. 2010-192483, filed on Aug. 30,2010, the entire contents of which are incorporated herein by reference.

FIELD

The embodiments discussed herein are related to a switching devicecompensation circuit.

BACKGROUND

In recent years, saving energy resources has become an important concernin various fields, and the field of power supplies, for example, is noexception. More specifically, there has developed, for example, a needto further enhance the efficiency of switching power supplies.

A switching power supply whose output efficiency exceeds 90% has alreadybeen proposed in the art, but the current state of the art isapproaching a limit when it comes to further enhancing the efficiency,because, for example, the power consumed by the switching transistor(switching device) used in the power supply becomes a bottleneck.

It is believed that the causes for the bottleneck due to the use of theswitching transistor are the parasitic resistive component called the ONresistance of the transistor, especially the component residing on thecurrent input side terminal of the transistor, and the capacitivecomponent seen between each terminal of the transistor.

First, the problem attributable to the parasitic resistive componentresiding on the current input side terminal of the transistor occurswhen the transistor is in the ON state. That is, when the transistor isturned on, allowing a current to flow through the transistor, the ONresistance of the transistor causes a voltage to develop between thecurrent-carrying terminals of the transistor due to the ON resistanceand the current in accordance with Ohm's law.

Here, since the power consumed by the transistor is equal to the productof the current flowing through the transistor and the voltage developedbetween the current-carrying terminals of the transistor, this power isnot one that is recoverable as the output of the switching power supply,but is converted in the transistor into heat, resulting in a power loss.

Next, the problem attributable to the capacitive component seen betweeneach terminal of the transistor occurs when the current and voltageabruptly change during the ON/OFF operation of the transistor. That is,during the ON/OFF operation of the transistor, the capacitance seenbetween each terminal of the transistor is charged and discharged.

Further, when the switching operation of the transistor starts, thecharging/discharging of the capacitance causes a delay in the timing ofswitching operation between the voltage and current of the transistor.The larger the capacitance, the greater the timing delay.

As a result, the voltage is applied before the current becomescompletely zero and, during this time, a power loss occurs, as in thecase of the problem attributable to the parasitic resistive componentresiding on the current input side terminal of the transistor.

Generally, in the switching power supply, a field-effect transistor(FET) has been used as the switching device, and a typical example ofsuch a transistor is a metal-oxide-semiconductor (MOS) transistor thatuses a silicon material. The power loss described above has been aserious problem with this type of MOS(metal-oxide-semiconductor(-semiconductor)) transistor.

To reduce the power loss, a transistor that does not use silicon butuses a compound semiconductor has been developed for use in a switchingpower supply. Since many of the compound semiconductors have greaterelectron mobility and larger mutual conductance than silicon, theadvantage is that not only is it possible to reduce the ON resistance,but the capacitance seen between each terminal of the transistor is alsosmall.

However, the electrical characteristics in steady-state switchingoperation of a field-effect transistor that uses a compoundsemiconductor may vary depending on the ambient temperature or on theapplied current and voltage; for example, the threshold voltage of thetransistor may vary greatly.

More specifically, the threshold voltage of an n-channel transistor usedin a switching power supply is normally expected to be positive, but inthe case of a transistor using a compound semiconductor, the thresholdvoltage may shift into the negative side, depending on the operatingconditions or operating environment.

The shift in the negative direction of the threshold voltage of such afield-effect transistor using a compound semiconductor occurs during theswitching operation of the transistor; it is said that this phenomenonis strongly dependent on the charge/discharge of electrons from theelectron trapping levels believed to exist at the semiconductor surface,the semiconductor-semiconductor interface, and thesemiconductor-insulator interface, but at the present time, the detailsof the cause are not fully understood, nor is it possible to completelycontrol the operation.

The variation of transistor threshold voltage occurs not only incompound semiconductor transistors, such as gallium-nitride highelectron mobility transistors (GaN HEMTs), but more or less in variousother transistors such as conventional MOS transistors.

The switching device compensation circuit according to any one of theembodiments described herein is widely applicable to various switchingdevices including compound semiconductor transistors such as GaN HEMTsand field-effect transistors such as MOSFETs.

Further, it will be appreciated that the switching device to becontrolled is not limited to the transistor used as the switching devicein the switching power supply, but may include switching devices used invarious other electrical circuits.

In the related art, various types of switching power supply apparatushave been proposed that use field-effect switching transistors and thatimprove efficiency by reducing losses during light load periods.

-   Patent Document 1: International Publication Pamphlet No. WO    2005/078910

SUMMARY

According to an aspect of the embodiments, a switching devicecompensation circuit performs switching control by applying a controlpulse to a control terminal of a switching device. The switching devicecompensation circuit includes a first threshold voltage change detectionunit, a first control signal generating unit, and an amplitude controlunit.

The first threshold voltage change detection unit detects a change inthreshold voltage of the switching device from an output voltagecontrolled via the switching device. The first control signal generatingunit generates a first control signal in accordance with an output ofthe first threshold voltage change detection unit. The amplitude controlunit controls the amplitude of the control pulse in accordance with anoutput of the first control signal generating unit.

The object and advantages of the embodiments will be realized andattained by means of the elements and combinations particularly pointedout in the claims.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory and arenot restrictive of the embodiments, as claimed.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram illustrating one example of a switching powersupply apparatus;

FIG. 2A, FIG. 2B, FIG. 2C, and FIG. 2D are diagrams (part 1) forexplaining how the threshold voltage of a switching device changes inthe switching power supply apparatus of FIG. 1;

FIG. 3A, FIG. 3B, FIG. 3C, and FIG. 3D are diagrams (part 2) forexplaining how the threshold voltage of the switching device changes inthe switching power supply apparatus of FIG. 1;

FIG. 4A, FIG. 4B, and FIG. 4C are diagrams for explaining the operationof the switching power supply apparatus of FIG. 1;

FIG. 5 is a block diagram illustrating one example of a switching powersupply apparatus to which a switching device compensation circuit of thepresent embodiment is applied;

FIG. 6 is a circuit diagram illustrating a specific example of theswitching power supply apparatus of FIG. 5;

FIG. 7 is a circuit diagram illustrating specifically a variable-gainamplifier incorporated in the switching power supply apparatus of FIG.6;

FIG. 8A and FIG. 8B are block diagrams for explaining modified examplesof the variable-gain amplifier of FIG. 7;

FIG. 9A and FIG. 9B are diagrams for explaining a sampling unit in theswitching power supply apparatus of FIG. 6;

FIG. 10 is a circuit diagram illustrating one example of the samplingunit of FIG. 9A;

FIG. 11A, FIG. 11B, and FIG. 11C are diagrams for explaining theoperation of the switching power supply apparatus of FIG. 6;

FIG. 12 is a block diagram illustrating a modified example of theswitching power supply apparatus of FIG. 5; and

FIG. 13 is a block diagram illustrating one example of a motor apparatusto which the switching power supply apparatus of FIG. 5 is applied.

DESCRIPTION OF EMBODIMENTS

Before describing embodiments of a switching device compensation circuitin detail, one example of a switching power supply apparatus will bedescribed with reference to FIG. 1, FIG. 2A to FIG. 2D, FIG. 3A to FIG.3D, and FIG. 4A to FIG. 4C.

FIG. 1 is a block diagram illustrating one example of a switching powersupply apparatus (power supply circuit) as a step-down switching powersupply apparatus.

In FIG. 1, reference numeral 101 is a switching transistor (switchingdevice), 102 is a current-detecting impedance device (impedance device),and 103 is a diode (diode device). Further, reference numeral 104 is aninductor (inductance device), 105 is a capacitor (smoothing capacitancedevice), and 106 is a load resistor.

On the other hand, reference numeral 107 is a voltage divider, 108 is areference power supply, 109 is a voltage/phase compensator, 110 is asawtooth wave signal source, 111 is a comparator and pulse generator,112 is a set/reset (SR) latch circuit, and 113 is a gate driver.Further, reference numeral 114 is a gate driver IC, 115 is a clock powersupply, 116 is an input power supply, and 117 is an error amplifier.

As illustrated in FIG. 1, in the step-down switching power supplyapparatus, the positive terminal of the input power supply 116 whosenegative terminal is grounded, and which provides an input voltage Vin,for example, is coupled to the drain of the switching transistor 101.

The source of the switching transistor 101 is coupled to one end of theimpedance device 102, and the other end of the impedance device 102 iscoupled to the anode of the diode 103 as well as to one end of theinductor 104.

The cathode of the diode 103 is grounded, while the other end of theinductor 104 is coupled to one end of the capacitor 105 as well as to anoutput voltage terminal that provides an output voltage Vout. The otherend of the capacitor 105 is grounded, and the load resistor 106 iscoupled between the output voltage terminal and ground.

Here, the impedance device 102 is provided to detect the current (coilcurrent) IL that flows via the switching transistor 101 to the inductor104, and the voltage developed across the impedance device 102 isdetected by a detector 124.

The voltage divider 107 constructed from series-connected resistors 171and 172 is coupled between the output voltage (Vout) terminal andground, and the error amplifier 117 compares the output voltage of thevoltage divider 107 with the voltage of the reference power supply 108to feedback-control the output voltage Vout.

The error amplifier 117 produces an output by amplifying the differencebetween the voltage of the reference power supply 108 applied to itspositive input (noninverting input) and the voltage divided between theresistors 171 and 172 in the voltage divider 107 and applied to itsnegative input (inverting input), and the output of the error amplifier117 is supplied to the voltage/phase compensator 109.

Here, the circuit is configured so that the output of the erroramplifier 117 is supplied via the voltage/phase compensator 109 to thecomparator and pulse generator 111, but in an alternative configuration,the voltage/phase compensator 109 may be inserted between the negativeinput terminal and output terminal of the error amplifier 117 inparallel therewith.

That is, the voltage/phase compensator 109 is a device for adjusting thespeed of the temporal change of the desired output voltage bycompensating the voltage and phase of the signal output from the erroramplifier 117, and the circuit may be implemented in various ways.

Besides the output of the voltage/phase compensator 109, the output ofthe detector 124 detecting the voltage developed across the impedancedevice 102 and the output of the sawtooth wave signal source 110generating a pulse wave are supplied as inputs to the comparator andpulse generator 111.

The comparator and pulse generator 111 compares (performs add andsubtract operations on) the various input signals and generates aprescribed pulse signal. The pulse signal generated by the comparatorand pulse generator 111 is applied to the reset terminal (R) of the SRlatch circuit 112.

Here, the output of the dedicated clock power supply 115 is applied tothe set terminal (S) of the SR latch circuit 112 in order to operate theSR latch circuit 112. As a result, the Q output of the SR latch circuit112 produces a pulse signal with a proper duty cycle, and this pulsesignal is supplied to the gate driver 113.

The output of the gate driver 113 is supplied to the gate of theswitching transistor 101 to control the switching on and off of thecurrent that flows between the drain and source of the switchingtransistor 101. In FIG. 1, the source of the switching transistor 101and ground are coupled to the gate driver 113.

Here, when a compound semiconductor transistor such as a GaN HEMT or afield-effect transistor such as a MOSFET is used as the switchingtransistor (switching device), the threshold voltage of the transistormay change, depending on the ambient temperature or on the appliedcurrent and voltage.

FIG. 2A, FIG. 2B, FIG. 2C, FIG. 2D, FIG. 3A, FIG. 3B, FIG. 3C, and FIG.3D are diagrams for explaining how the threshold voltage of theswitching device changes in the switching power supply apparatus ofFIG. 1. FIG. 2A to FIG. 2D illustrate the case where the thresholdvoltage Vth changes in the positive direction, and FIG. 3A to FIG. 3Dillustrate the case where the threshold voltage Vth changes in thenegative direction.

While FIG. 2A to FIG. 2D and FIG. 3A to FIG. 3D both illustrate thechange in the threshold voltage Vth for the case where an n-channel MOStransistor or a GaN HEMT is used as the switching device, it will berecognized that in the case of other types of switching device also, itsthreshold voltage may change in like manner depending on the ambienttemperature or on the applied voltage, etc.

First, referring to FIG. 2A to FIG. 2D, a description will be given ofthe case where the threshold voltage Vth of the switching device(switching transistor) changes in the positive direction.

FIG. 2A illustrates the relationship between the drain-to-source voltageVds and the drain-to-source current Ids (IV characteristics) for variousgate-to-source voltages Vgs of the switching transistor; here, referencecharacter LL1 indicates the load line.

FIG. 2C illustrates the relationship between Vgs and Ids when thethreshold voltage of the switching transistor is Vth11, and referencecharacter CL11 indicates the characteristic curve of the switchingtransistor at that time. Reference character PA1 indicates a controlpulse with a voltage amplitude Vp1 that is applied to the gate of theswitching transistor.

FIG. 2D illustrates the relationship between Vgs and Ids when thethreshold voltage of the switching transistor changes from Vth11 in FIG.2C to Vth12 in the positive direction by ΔVth1, and reference characterCL12 indicates the characteristic curve of the switching transistor atthat time.

As is apparent from a comparison between FIG. 2C and FIG. 2D, when thethreshold voltage changes in the positive direction by ΔVth1, thecharacteristic curve CL12 of the switching transistor after the changeis given by shifting its initial characteristic curve CL11 in thepositive direction by ΔVth1.

As illustrated in FIG. 2D, when the threshold voltage changes from Vth11to Vth12 in the positive direction by ΔVth1, the source-to-drain currentIds decreases by ΔIds1. Further, as illustrated in FIG. 2B, thedrain-to-source voltage Vds for each gate-to-source voltage Vgsincreases by a given amount (for example, by ΔVds1 when Vgs=10 V).

That is, in the switching power supply apparatus of FIG. 1, if thethreshold voltage Vth of the switching device changes, for example, inthe positive direction during operation, it becomes difficult with thecontrol pulse PA1 to control the output voltage to the desired voltagevalue, and the power loss associated with the switching deviceincreases.

Next, referring to FIG. 3A to FIG. 3D, a description will be given ofthe case where the threshold voltage Vth of the switching transistorchanges in the negative direction.

FIG. 3B illustrates the relationship between the drain-to-source voltageVds and the drain-to-source current Ids (IV characteristics) for variousgate-to-source voltages Vgs of the switching transistor; here, referencecharacter LL2 indicates the load line.

FIG. 3D illustrates the relationship between Vgs and Ids when thethreshold voltage of the switching transistor is Vth21 (for example,Vth21=0 V), and reference character CL21 indicates the characteristiccurve of the switching transistor at that time. Reference character PA2indicates a control pulse with a voltage amplitude Vp2 that is appliedto the gate of the switching transistor.

FIG. 3C illustrates the relationship between Vgs and Ids when thethreshold voltage of the switching transistor changes from Vth21 in FIG.3D to Vth22 in the negative direction by ΔVth2, and reference characterCL22 indicates the characteristic curve of the switching transistor atthat time.

Here, FIG. 2A, FIG. 2B, FIG. 2C, and FIG. 2D given above to explain thecase where the threshold voltage Vth changes in the positive directioncorrespond to FIG. 3B, FIG. 3A, FIG. 3D, and FIG. 3C, respectively,given here to explain the case where the threshold voltage Vth changesin the negative direction, though the scale used to plot the graph isdifferent between them.

As is apparent from a comparison between FIG. 3D and FIG. 3C, when thethreshold voltage changes in the negative direction by ΔVth2, thecharacteristic curve CL22 of the switching transistor after the changeis given by shifting its initial characteristic curve CL21 in thenegative direction by ΔVth2.

This means that even when the control pulse PA2 is not applied to thegate of the switching transistor (that is, Vgs=0 V), for example, theswitching transistor turns on and the drain-to-source current Ids flows(does not switch off), as illustrated in FIG. 3A.

That is, in the switching power supply apparatus of FIG. 1, if thethreshold voltage Vth of the switching device changes, for example, inthe negative direction during operation, the power loss increases andthe output efficiency greatly drops. In other words, the temperaturerise in the switching transistor may become excessive and may eventuallylead to the breakdown of the switching transistor.

FIG. 4A, FIG. 4B, and FIG. 4C are diagrams for explaining the operationof the switching power supply apparatus of FIG. 1 and illustrate thesimulation results obtained by sinusoidally varying the thresholdvoltage Vth between +5 V and −5 V. FIG. 4A illustrates the variation ofthe threshold voltage Vth with time T, FIG. 4B illustrates the variationof the drain-to-source current Ids with time T in that case, and FIG. 4Cillustrates the variation of the output voltage Vout with time T.

As illustrated in FIG. 4A to FIG. 4C, when the threshold voltage Vthchanges as depicted in FIG. 4A, the drain-to-source current Ids changesas depicted in FIG. 4B and the output voltage Vout changes as depictedin FIG. 4C.

That is, when the threshold voltage Vth of the switching device changesin the positive direction, it is not possible to turn on the switchingdevice with the control pulse PA1 of a constant voltage amplitude (Vp1),resulting in a situation where it is not possible to flow the currentIds.

When the threshold voltage Vth of the switching device changes in thenegative direction, the switching device is always ON with the controlpulse PA2 of a constant voltage amplitude (Vp2), resulting in asituation where the current Ids continues to flow.

In this way, when the threshold voltage Vth of the switching devicechanges in the positive or negative direction, it is not possible tomaintain the output voltage Vout at the desired voltage (for example,about 40 V), and the output voltage Vout varies greatly by more than100%. Furthermore, the power loss due to the switching device increases,and there may also arise the possibility of the switching devicebreaking down.

Embodiments of the switching device compensation circuit will bedescribed in detail below with reference to the accompanying drawings.

FIG. 5 is a block diagram illustrating one example of a switching powersupply apparatus (power supply circuit) as a step-down switching powersupply apparatus to which the switching device compensation circuit ofthe present embodiment is applied.

In FIG. 5, reference numeral 1 is a switching transistor (switchingdevice), 2 is a current-detecting impedance device (impedance device),and 3 is a diode (diode device). Reference numeral 4 is an inductor(inductance device), 5 is a capacitor (smoothing capacitance device), 6is a load resistor, and 7 is a voltage divider.

Further, reference numeral 8 is a reference power supply (firstreference power supply), 9 is a voltage/phase compensator (firstvoltage/phase compensator), 10 is a sawtooth wave signal source (firstsawtooth wave signal source), 11 is a comparator and pulse generator(first comparator and pulse generator), and 16 is an input power supply.

Reference numeral 17 is an error amplifier (first error amplifier), 18is a filter (first filter), 19 is a buffer amplifier (first bufferamplifier), 20 is a converter (first converter), 21 is a variable-gainamplifier, 22 is a reference power supply (second reference powersupply), and 24 is a detector.

Further, reference numeral 23 is a sawtooth wave signal source (secondsawtooth wave signal source), 25 is a sampling unit, 26 is an erroramplifier (second error amplifier), 27 is a comparator and pulsegenerator (second comparator and pulse generator), 28 is a switch, and29 is a DC voltage level power supply.

As is apparent from a comparison between FIG. 5 and the previously givenFIG. 1, the step-down switching power supply apparatus of FIG. 5 differsfrom the power supply apparatus of FIG. 1 by the inclusion of a circuitfor compensating the amplitude of the control pulse to be supplied tothe gate of the switching transistor 1.

As illustrated in FIG. 5, in the step-down switching power supplyapparatus, the positive terminal of the input power supply 16 whosenegative terminal is grounded, and which provides an input voltage Vin,for example, is coupled to the drain of the switching transistor 1.

The source of the switching transistor 1 is coupled to one end of theimpedance device 2, and the other end of the impedance device 2 iscoupled to the anode of the diode 3 as well as to one end of theinductor 4.

The cathode of the diode 3 is grounded, while the other end of theinductor 4 is coupled to one end of the capacitor 5 as well as to anoutput voltage terminal that provides an output voltage Vout.

The other end of the capacitor 5 is grounded, and the load resistor 6 iscoupled between the output voltage terminal and ground. Here, atransistor whose switching operation is controllable may be substitutedfor the diode (diode device) 3 in order to achieve synchronousrectification in the switching power supply apparatus.

The impedance device 2 is provided to detect the current (coil current)IL that flows via the switching transistor 1 to the inductor 4, and thevoltage developed across the impedance device 2 is detected by thedetector 24.

The voltage divider 7 constructed from series-connected resistors 71 and72 is coupled between the output voltage (Vout) terminal and ground, andthe error amplifier 17 compares the output voltage of the voltagedivider 7 with the voltage of the reference power supply 8 tofeedback-control the output voltage Vout.

The error amplifier 17 produces an output by amplifying the differencebetween the voltage of the reference power supply 8 applied to itspositive input (noninverting input) and the voltage divided between theresistors 71 and 72 in the voltage divider 7 and applied to its negativeinput (inverting input), and the output of the error amplifier 17 issupplied to the voltage/phase compensator 9.

Here, the circuit is configured so that the output of the erroramplifier 17 is supplied via the voltage/phase compensator 9 to both thecomparator and pulse generator 11 and the filter 18, but in analternative configuration, the voltage/phase compensator 9 may beinserted between the negative input terminal and output terminal of theerror amplifier 17 in parallel therewith.

That is, the voltage/phase compensator 9 is a device for adjusting thespeed of the temporal change of the desired output voltage bycompensating the voltage and phase of the signal output from the erroramplifier 17, and the circuit may be implemented in various ways.

More specifically, rather than inserting the voltage/phase compensator 9at the output of the error amplifier 17, various other implementationsare possible; for example, it may be inserted between the negative inputterminal and output terminal of the error amplifier 17 in paralleltherewith, or alternatively, it may be inserted between ground and thenegative input terminal of the error amplifier 17.

The output of the voltage/phase compensator 9 is supplied to the filter18 as well as to one input of the comparator and pulse generator 11whose other input is coupled to the output of the sawtooth wave signalsource 10 that generates a pulse wave.

The output of the comparator and pulse generator 11 is supplied to thevariable-gain amplifier 21, while on the other hand, the output of theerror amplifier 17, which is passed through the voltage/phasecompensator 9 and the filter 18, is amplified by the buffer amplifier 19and then supplied via the converter 20 to the variable-gain amplifier21.

Here, the filter 18, the buffer amplifier 19, and the converter 20together correspond to a first control signal generating unit thatgenerates a first control signal for adjusting the amplitude of thecontrol pulse to be supplied via the variable-gain amplifier 21 to thegate of the switching transistor 1.

The first control signal generating unit generates the first controlsignal for adjusting the amplitude of the control pulse to be suppliedto the gate of the switching transistor 1 in accordance with the changein the threshold voltage of the switching transistor 1 detected from theoutput voltage Vout of the switching power supply apparatus.

This means that the amplitude of the output of the comparator and pulsegenerator 11 supplied to the variable-gain amplifier 21 (the amplitudeof the control pulse to be supplied to the gate of the switchingtransistor 1) is adjusted, for example, by the output of the firstcontrol signal generating unit (the converter 20).

On the other hand, the voltage developed across the impedance device 2and detected by the detector 24 is supplied to the sampling unit 25 tobe described later, and the output of the sampling unit 25 is applied tothe positive input terminal of the error amplifier 26.

The error amplifier 26 amplifies a difference voltage corresponding tothe difference between the output of the reference power supply 22applied to its negative input terminal and the output of the samplingunit 25 applied to its positive input terminal, and supplies thedifference voltage to the comparator and pulse generator 27.

The comparator and pulse generator 27 is also supplied with the outputof the sawtooth wave signal source 23, and the output of the comparatorand pulse generator 27 is used to control the opening and closing of theswitch 28 coupled to the DC voltage level power supply 29.

Here, the reference power supply 22, the sawtooth wave signal source 23,the error amplifier 26, and the comparator and pulse generator 27together correspond to a second control signal generating unit.

The second control signal generating unit generates a second controlsignal in accordance with the change in the threshold voltage of theswitching transistor 1 detected from the current flowing through theswitching transistor 1 (the voltage developed across the impedancedevice 2).

The second control signal is used to adjust the amplitude of the controlpulse to be supplied to the gate of the switching transistor 1.

More specifically, the DC voltage level to be applied to thevariable-gain amplifier 21 (the offset voltage for the control pulse tobe supplied to the gate of the switching transistor 1) is adjusted bycontrolling the switch 28 in accordance with the output of the secondcontrol signal generating unit (the output of the comparator and pulsegenerator 27).

The switching device compensation circuit of the present embodiment isnot limited in its application to the switching transistor for use inthe step-down switching power supply apparatus, but is widely applicableto various circuits that use switching devices whose threshold voltagestend to vary.

That the threshold voltage of the transistor varies (changes) meanseither (1) that the desired current does not flow at the gate voltagethat drives the transistor or (2) that the current (Ids) flowing throughthe transistor does not become zero during the on/off operation of thegate.

First, the above situation (1) occurs in many cases when the thresholdvoltage of the transistor (switching transistor 1) is shifted in thepositive direction from the initial state. To correct this situation,the DC gate voltage of the transistor, for example, need only be shiftedin the positive direction to increase the pulse voltage (the amplitudeof the control pulse) to be applied to the gate of the transistor.

The first control signal generating unit accomplishes this; that is, thefirst control signal is generated from the output of the error amplifier17 (the voltage/phase compensator 9) by passing the output through thefilter 18, the buffer amplifier 19, and the converter 20, and this firstcontrol signal is supplied to the gain adjusting terminal of thevariable-gain amplifier 21.

Here, the output voltage Vout of the switching power supply apparatus isdivided by the voltage divider 7, and the divided voltage is supplied tothe error amplifier 17 where it is compared with the output voltage ofthe reference power supply 8.

In this case, if the output voltage Vout is not equal to the desiredvoltage, the output voltage of the error amplifier 17 does not becomezero, but produces an output voltage of a certain level. This outputvoltage level of the error amplifier 17 is not proportional to thethreshold voltage Vth.

In view of this, the converter 20 is provided to convert the outputvoltage level of the error amplifier 17 into a suitable value, and theoutput of the converter 20 is used to control the gain of thevariable-gain amplifier 21 and thereby control the amplitude of thecontrol pulse to be supplied to the switching transistor 1. Thus, theswitching transistor 1 is controlled so that the current Ids flowingthrough the transistor 1 is maintained at the proper value.

In this way, the first control signal generating unit generates thefirst control signal for adjusting the amplitude of the control pulse tobe supplied to the gate of the switching transistor 1 in accordance withthe output voltage Vout of the switching power supply apparatus.

Of course, in addition to controlling the gain of the variable-gainamplifier 21 by the converter 20, the offset voltage may be adjusted byshifting the DC voltage level of the control pulse to be supplied to thegate of the switching transistor 1.

Next, the above situation (2) occurs in many cases when the thresholdvoltage of the transistor (switching transistor 1) is shifted in thenegative direction from the initial state. To correct this situation,the DC gate voltage of the transistor, for example, need only be shiftedin the negative direction so that any DC current does not flow when thepulse voltage applied to the gate of the transistor is 0 V.

That is, when the voltage of the control pulse applied to the gate ofthe transistor is 0 V, the DC component of the drain current Ids of thetransistor is monitored, and control is performed so that the current isconstantly held at 0.

The second control signal generating unit accomplishes this; that is, acomparison is made by the error amplifier 26 to see if the signal outputfrom the sampling unit 25 is identical with the desired signal level(the output voltage of the reference power supply 22), and the output ofthe error amplifier 26 is supplied to the comparator and pulse generator27.

The comparator and pulse generator 27 generates the second controlsignal from the output of the error amplifier 26 and the output of thesawtooth wave signal source 23, and the opening and closing of theswitch 28 is controlled by the second control signal.

Here, the switch 28 is used to control the level of the DC voltage to besupplied to the variable-gain amplifier 21, and selects one or the otherof the voltages at the two ends of the DC voltage level power supply 29and supplies the selected one to the variable-gain amplifier 21.

The detector 24 detects the voltage developed across the impedancedevice 2, and the DC component (bottom-hold value) of the signal outputfrom the detector 24 is extracted by the sampling unit 25.

In this way, the second control signal generating unit generates thesecond control signal for adjusting the DC voltage level of the controlpulse to be supplied to the gate of the switching transistor 1 inaccordance with the voltage developed across the impedance device 2,that is, the current Ids flowing through the transistor 1.

FIG. 6 is a circuit diagram illustrating a specific example of theswitching power supply apparatus of FIG. 5; here, the voltage/phasecompensator 9, the filter 18, the converter 20, the variable-gainamplifier 21, etc. are each depicted in the form of a specific circuit.In the circuit of FIG. 6, the voltage/phase compensator 9 is not placedafter the error amplifier 17 but inserted between the negative inputterminal and output terminal of the error amplifier 17.

As illustrated in FIG. 6, the voltage/phase compensator 9 includes aseries coupling of a resistor 91 and a capacitor 92, and a capacitor 93,both inserted between the negative input terminal and output terminal ofthe error amplifier 17. The voltage/phase compensator 9 adjusts thespeed of the temporal change of the desired output voltage and thuscontrols the switching power supply apparatus so that the entireoperation of the apparatus does not become unstable.

The voltage/phase compensator 9 may be implemented in various ways; forexample, it may be placed between the output of the error amplifier 17and the input of the comparator and pulse generator 11, as depicted inFIG. 5, or alternatively, it may be inserted between ground and thenegative input terminal of the error amplifier 17.

The filter 18 includes an inductor 181 and a capacitor 182, and has thefunction of bringing the output signal of the error amplifier 17 closerto the value of the DC. The output of the filter 18 is coupled to thenegative input of the buffer amplifier 19 whose positive input isgrounded, and the output level of the filter 18 thus adjusted by thebuffer amplifier 19 is supplied to the converter 20.

The converter 20 may be constructed, for example, as a voltageconversion circuit using a field-effect transistor, and includes anamplifier 201, a transistor 202, and a resistor 203. The converter 20converts the output of the error amplifier 17, supplied via the filter18 and the buffer amplifier 19, into a suitable voltage or voltagefunction, and supplies the thus converted output to the gain adjustingterminal of the variable-gain amplifier 21.

The variable-gain amplifier 21 includes resistors R1, R2, R3, and R4,and an amplifier 210. The resistor R3 is a variable resistor whoseresistance value is controlled by the output of the converter 20 (thefirst control signal generating unit) thereby to adjust the amplitude ofthe output of the comparator and pulse generator 11, that is, theamplitude of the control pulse to be supplied to the gate of theswitching transistor 1.

Connected to the variable-gain amplifier 21 are the output terminal ofthe comparator and pulse generator 11, the negative potential terminalof the DC voltage level power supply 29, and the switching terminal ofthe switch 28 which selects the positive or negative potential terminalof the DC voltage level power supply 29 for coupling.

Here, the negative potential terminal of the DC voltage level powersupply 29 is coupled to the source of the switching transistor 1, sothat the voltage of the DC voltage level power supply 29 is at the samepotential as the source voltage Vs of the switching transistor 1.

Further, the switch 28 selects, under the control of the output of thecomparator and pulse generator 27, the negative potential (the sourcevoltage Vs of the switching transistor 1) or the positive potential ofthe DC voltage level power supply 29, and the selected DC voltage levelis supplied to the variable-gain amplifier 21.

The switching terminal of the switch 28 is coupled to the positive inputof the amplifier 210 via the resistor R4, and the output of theamplifier 210 is coupled to the gate of the switching transistor 1.

The resistor R2 is coupled between the negative input terminal andoutput terminal of the amplifier 210, and the resistance value of theresistor R3 is variable under the control of the output of thecomparator and pulse generator 11; similarly, the resistance value ofthe resistor R4 is variable under the control of the output of theconverter 20 (the amplifier 201).

Here, with the switch 28 controlled by the output of the comparator andpulse generator 27 (the second control signal from the second controlsignal generating unit), the offset voltage is adjusted by shifting theDC voltage level of the control pulse to be supplied to the gate of theswitching transistor 1.

As will be described hereinafter with reference to FIG. 7, the offsetvoltage may also be adjusted by controlling the resistance value of theresistor R4 as a variable resistor by the output (second control signal)of the second control signal generating unit.

FIG. 7 is a circuit diagram illustrating specifically the variable-gainamplifier incorporated in the switching power supply apparatus of FIG.6. As illustrated in FIG. 7, the variable-gain amplifier 21 includes theresistors R1, R2, R3, and R4, and the amplifier 210, and the resistorsR3 and R4 are variable resistors.

Here, the amplitude of the signal supplied via a terminal 215 to thepositive input terminal of the amplifier 210 (i.e., the output signalfrom the comparator and pulse generator 11) is, for example, adjusted inaccordance with the resistance value of the variable resistor R3 whichis controlled by the output of the converter 20 (the first controlsignal generating unit) supplied via a terminal 214. In this way, theamplitude of the control pulse, which is output via a terminal 211 andsupplied to the gate of the switching transistor 1, is adjusted.

On the other hand, the DC voltage level applied via a terminal 213 is,for example, adjusted in accordance with the resistance value of thevariable resistor R4 which is controlled by the output of the comparatorand pulse generator 27 (the second control signal generating unit). Inthis way, the offset voltage for the control pulse, which is output viathe terminal 211 and supplied to the gate of the switching transistor 1,is adjusted.

The specific circuit configuration of the variable-gain amplifier 21 andthe control method thereof may be variously modified; for example, inthe example previously described with reference to FIG. 6, the DCvoltage level to be applied via the terminal 213 is directly controlledby the switch 28. In the example of FIG. 6, the source voltage Vs of theswitching transistor 1 is applied to a terminal 216.

FIG. 8A and FIG. 8B are block diagrams for explaining modified examplesof the variable-gain amplifier of FIG. 7. As described above, thevariable-gain amplifier 21 is configured to be able to simultaneouslyadjust both the amplitude (gain) of the control pulse to be supplied tothe gate of the switching transistor 1 and the offset voltage thereof(the DC voltage level).

However, these functions may be implemented using separate circuitblocks, as illustrated in FIG. 8A or FIG. 8B. Here, reference numeral 21a is a variable-gain amplifier block for adjusting the amplitude of thecontrol pulse, and 21 b is a DC voltage level correcting block (levelshift circuit) for adjusting the offset voltage for the control pulse.

That is, as illustrated in FIG. 8A, the input signal Sin may first bepassed, for example, through the variable-gain amplifier block 21 awhich adjusts the gain in accordance with a control signal CSa, and thenpassed through the DC voltage level correcting block 21 b which adjuststhe DC voltage level in accordance with a control signal CSb.

Alternatively, as illustrated in FIG. 8B, the input signal Sin may firstbe passed, for example, through the DC voltage level correcting block 21b which adjusts the DC voltage level in accordance with the controlsignal CSb, and then passed through the variable-gain amplifier block 21a which adjusts the gain in accordance with the control signal CSa.

That is, the amplitude (gain) of the control pulse to be supplied to thegate of the switching transistor 1 and the offset voltage thereof (theDC voltage level) may be adjusted simultaneously or separately in anyorder, by using the same or separate circuit blocks.

FIG. 9A and FIG. 9B are diagrams for explaining the sampling unit in theswitching power supply apparatus of FIG. 6; FIG. 9A is a block diagramof the sampling unit, and FIG. 9B is a diagram for explaining theoperation of the sampling unit.

As illustrated in FIG. 9A, the sampling unit 25 includes an AC componentextracting unit 51, a rectifying unit 52, a voltage smoothing unit 53,adders 54 and 55, and a voltage averaging unit 56.

The AC component extracting unit 51 extracts the AC component from theinput signal S1 (i.e., the output signal of the detector 24), andsupplies it to the rectifying unit 52 and also to the negative input ofthe adder 55. The rectifying unit 52 rectifies the AC componentextracted by the AC component extracting unit 51 and supplies its outputto the voltage smoothing unit 53.

The adder 55 subtracts from the DC component of the input signal 51 theAC component extracted by the AC component extracting unit 51, andsupplies the result to the voltage averaging unit 56. The adder 54subtracts the output of the voltage smoothing unit 53 from the output ofthe voltage averaging unit 56, and outputs a signal S2.

That is, as illustrated in FIG. 9B, the sampling unit 25 functions as avalley-hold circuit that holds a voltage Vv corresponding to the minimumvalue (valley) Pv of the AC input signal S1 on which the offset voltageis superimposed. In FIG. 9B, reference character Pp indicates themaximum value (peak) of the input signal S1.

FIG. 10 is a circuit diagram illustrating one example of the samplingunit of FIG. 9A. As illustrated in FIG. 10, the sampling unit 25includes capacitors 251, 255, and 259, buffer amplifiers 252 and 257, adiode 253, a resistor 254, an inductor 258, and a buffering differentialamplifier 256.

As is apparent from a comparison between FIG. 10 and FIG. 9A, the ACcomponent extracting unit 51 corresponds to the capacitor 251 and bufferamplifier 252, the rectifying unit 52 corresponds to the diode 253, andthe voltage smoothing unit 53 corresponds to the resistor 254 andcapacitor 255.

Further, the adder 54 corresponds to the buffering differentialamplifier 256, the adder 55 corresponds to the buffer amplifier 257, andthe voltage averaging unit 56 corresponds to the inductor 258 andcapacitor 258. Thus, the sampling unit 25 of FIG. 9A may be implementedusing the circuit depicted in FIG. 10.

More specifically, first the input signal S1 is separated into its ACand DC components, and the AC component is supplied via the capacitor251 to the positive input terminal of the buffer amplifier 252. Thenegative input terminal of the buffer amplifier 252 is coupled to itsoutput terminal.

The DC component of the input signal S1 is supplied to the positiveinput terminal of the buffer amplifier 257, while the output signal fromthe buffer amplifier 252 is supplied to the negative input terminal ofthe buffer amplifier 257. The output signal from the buffer amplifier257 is supplied via the inductor 258 to the positive input terminal ofthe buffering differential amplifier 256.

The capacitor 259 is provided between ground and the positive inputterminal of the buffering differential amplifier 256, and the voltageaveraged through a low-pass filter formed by the inductor 258 andcapacitor 259 is supplied to the positive input terminal of thebuffering differential amplifier 256.

On the other hand, the output signal from the buffer amplifier 252 isrectified (half-wave rectified) by the diode 253 and supplied to thenegative input terminal of the buffering differential amplifier 256. Theresistor 254 and capacitor 255 are provided between ground and thenegative input terminal of the buffering differential amplifier 256 sothat the signal rectified by the diode 253 is smoothed before beingsupplied to the negative input terminal of the buffering differentialamplifier 256.

Here, the diode 253 detects the valley (peak) voltage of the AC inputsignal S1, and this valley voltage is smoothed before being supplied tothe negative input terminal of the buffering differential amplifier 256.

The buffering differential amplifier 256 receives at its positiveterminal the output signal of the buffer amplifier 257 averaged throughthe low-pass filter formed by the inductor 258 and capacitor 259, andamplifies the difference between the averaged voltage signal and thesmoothed valley voltage signal received at its negative input terminal.

Thus, the buffering differential amplifier 256 outputs the signal S2representing the minimum DC signal component (valley-hold voltage) ofthe input signal S1 from which the AC component has been removed.

FIG. 11A, FIG. 11B, and FIG. 11C are diagrams for explaining theoperation of the switching power supply apparatus of FIG. 6 andillustrate the simulation results obtained by sinusoidally varying thethreshold voltage Vth between +5 V and −5 V. FIG. 11A illustrates thevariation of the threshold voltage Vth with time T, FIG. 11B illustratesthe variation of the drain-to-source current Ids with time T in thatcase, and FIG. 11C illustrates the variation of the output voltage Voutwith time T.

As illustrated in FIG. 11A to FIG. 11C, when the threshold voltage Vthchanges as depicted in FIG. 11A, the drain-to-source current Ids changesas depicted in FIG. 11B and the output voltage Vout changes as depictedin FIG. 11C.

That is, whether the threshold voltage Vth of the switching devicechanges in the positive direction or negative direction, a control pulseof substantially constant voltage amplitude is supplied to the switchingdevice as depicted in FIG. 11B, and thus the output voltage Vout ismaintained at the desired voltage (about 40 V).

More specifically, as may be seen from FIG. 11C, when the output voltageis 39.8 V, for example, even if the threshold voltage Vth changes asdepicted in FIG. 11A, it is possible to hold the variation of the outputvoltage Vout to within about 5%.

This means that the power associated with the switching device is small,making it possible, for example, to further enhance the efficiency ofthe switching power supply apparatus.

FIG. 12 is a block diagram illustrating a modified example of theswitching power supply apparatus of FIG. 5. As is apparent from acomparison between FIG. 12 and FIG. 5, the second control signalgenerating unit in this modified example is identical in configurationto the first control signal generating unit.

That is, the second control signal generating unit for adjusting theoffset voltage for the control pulse to be supplied to the gate of theswitching transistor 1 includes a filter 31, a buffer amplifier 32, anda converter 33.

In the modified example, as in the embodiment of FIG. 5, the firstcontrol signal generating unit for adjusting the amplitude of thecontrol pulse to be supplied to the gate of the switching transistor 1includes the filter 18, the buffer amplifier 19, and the converter 20.

More specifically, the output of the sampling unit 25 is supplied to avoltage/phase compensator 30 which adjusts the speed of the temporalchange of the current, and the output of the voltage/phase compensator30 is supplied to the filter 31 where high-frequency components areeliminated from that output.

It will be appreciated that the voltage/phase compensator 30 may bevariously modified, as in the case of the previously describedvoltage/phase compensator 9, and is not limited to the one insertedbetween the sampling unit 25 and the filter 31.

The output of the filter 31 is supplied to the buffer amplifier 32, andthe output of the buffer amplifier 32 is supplied to the variable-gainamplifier 21 via the converter 33 for converting that output into anysuitable output (prescribed level).

More specifically, the converter 33 supplies the output of the bufferamplifier 32 to the variable-gain amplifier 21 after converting it, forexample, into an output such that the DC component flowing through theswitching transistor 1 becomes zero. This makes it possible to set theDC gate voltage of the switching transistor 1 properly, for example,even when the threshold voltage of the switching transistor 1 is shiftedin the positive direction.

In the second control signal generating unit of the modified example,after the voltage and phase of the signal output from the sampling unit25 has been compensated by the voltage/phase compensator 30, the outputis passed through the filter 31 and amplified by the buffer amplifier 32whose output is then converted by the converter 33 into a prescribedsignal (second control signal) for output.

Here, the output signal from the converter 33 is applied, for example,to the terminal 212 in FIG. 7 to control the resistance value of thevariable resistor R4 and thereby adjust the DC voltage level applied tothe variable-gain amplifier 21. In this way, the offset voltage for thecontrol pulse to be supplied to the gate of the switching transistor 1is adjusted.

On the other hand, the output signal from the converter 20 is applied,for example, to the terminal 214 in FIG. 7 to control the resistancevalue of the variable resistor R3 and thereby adjust the amplitude ofthe output signal of the comparator and pulse generator 11. In this way,the amplitude of the control pulse to be supplied to the gate of theswitching transistor 1 is adjusted.

While the above embodiment and modified example have been described asincluding both the first control signal generating unit and the secondfirst control signal generating unit, it is possible to achieve theeffect of reducing the power loss associated with the switchingtransistor 1 if one or the other of them is omitted.

That is, by just adjusting either the offset voltage or the amplitude ofthe control pulse to be supplied to the gate of the switching transistor1, it is possible to achieve a reduction in power loss. In other words,if one or the other of the first control signal generating unit and thesecond first control signal generating unit is omitted from theswitching device compensation circuit, it is possible to reduce thepower loss associated with the switching device.

FIG. 13 is a block diagram illustrating one example of a motor apparatusto which the switching power supply apparatus of FIG. 5 is applied;here, a DC motor 300 is provided as the load resistor 6 and the DC motor300 is driven by the output voltage of the switching power supplyapparatus.

As illustrated in FIG. 13, the switching device compensation circuitdescribed above is also applicable, for example, to the motor apparatusthat drives the DC motor 300. Furthermore, the switching devicecompensation circuit of each of the embodiments described herein iswidely applicable as a switching device compensation circuit or the likefor use in a known motor inverter capable of varying the number ofrevolutions or the rotational torque of an AC motor (for example, athree-phase motor) as desired.

Furthermore, as previously described, the switching device compensationcircuit of each of the embodiments described herein is widely applicableto various switching devices including compound semiconductortransistors such as GaN HEMTs and field-effect transistors such asMOSFETs.

All examples and conditional language recited herein are intended forpedagogical purposes to aid the reader in understanding the inventionand the concepts contributed by the inventor to furthering the art, andare to be construed as being without limitation to such specificallyrecited examples and conditions, nor does the organization of suchexamples in the specification relate to a showing of the superiority andinferiority of the invention. Although the embodiments of the presentinvention have been described in detail, it should be understood thatthe various changes, substitutions, and alterations could be made heretowithout departing from the spirit and scope of the invention.

What is claimed is:
 1. A switching device compensation circuit, whichperforms switching control by applying a control pulse to a controlterminal of a switching device, the switching device compensationcircuit comprising: a first threshold voltage change detection unitconfigured to detect a change in threshold voltage of the switchingdevice from an output voltage controlled via the switching device; afirst control signal generating unit configured to generate a firstcontrol signal in accordance with an output of the first thresholdvoltage change detection unit; and an amplitude control unit including avariable-gain amplifier configured to control an amplitude of thecontrol pulse to be applied to the control terminal of the switchingdevice in accordance with the first control signal output from the firstcontrol signal generating unit.
 2. The switching device compensationcircuit as claimed in claim 1, wherein the first control signalgenerating unit includes: a first filter configured to filter the outputof the first threshold voltage change detection unit; a first bufferamplifier configured to buffer and amplifies an output of the firstfilter; and a first converter configured to output the first controlsignal by converting an output of the first buffer amplifier, andwherein a gain in the variable-gain amplifier is adjusted by the firstcontrol signal supplied from the first converter.
 3. A switching devicecompensation circuit, which performs switching control by applying acontrol pulse to a control terminal of a switching device, the devicecompensation circuit comprising: a first threshold voltage changedetection unit configured to detect a change in threshold voltage of theswitching device from an output voltage controlled via the switchingdevice; a first control signal generating unit configured to generate afirst control signal in accordance with an output of the first thresholdvoltage change detection unit; and an amplitude control unit configuredto control the amplitude of the control pulse in accordance with anoutput of the first control signal generating unit, wherein the firstthreshold voltage change detection unit includes a first error amplifierconfigured to compare a voltage obtained by dividing the output voltagewith a voltage supplied from a first reference power supply.
 4. Aswitching device compensation circuit, which performs switching controlby applying a control pulse to a control terminal of a switching device,the switching device compensation circuit comprising: a second thresholdvoltage change detection unit configured to detect a change in thresholdvoltage of the switching device from a current flowing through theswitching device; a second control signal generating unit configured togenerate a second control signal in accordance with an output of thesecond threshold voltage change detection unit; and an offset voltagecontrol unit including a variable-gain amplifier configured to controlan offset voltage for the control pulse to be applied to the controlterminal of the switching device in accordance with the second controlsignal output from the second control signal generating unit.
 5. Theswitching device compensation circuit as claimed in claim 4, wherein thesecond control signal generating unit includes: a second error amplifierconfigured to compare a voltage output from the second threshold voltagechange detection unit with a voltage supplied from a second referencepower supply; and a second comparator and pulse generator configured tooutput the second control signal of a prescribed pulse in accordancewith an output of the second error amplifier, and wherein a DC voltagelevel to be applied to the variable-gain amplifier is adjusted by thesecond control signal supplied from the second comparator and pulsegenerator.
 6. The switching device compensation circuit as claimed inclaim 5, wherein the offset voltage control unit further includes aswitch whose switching is controlled by the second control signalsupplied from the second comparator and pulse generator, and whichapplies a DC voltage of a different level to the variable-gainamplifier.
 7. The switching device compensation circuit as claimed inclaim 4, wherein the second control signal generating unit includes: asecond filter configured to filter the output of the second thresholdvoltage change detection unit; a second buffer amplifier configured tobuffer and amplifies an output of the second filter; and a secondconverter configured to output the second control signal by convertingan output of the second buffer amplifier, and wherein the DC voltagelevel in the variable-gain amplifier is adjusted by the second controlsignal supplied from the second converter.
 8. A switching devicecompensation circuit, which performs switching control by applying acontrol pulse to a control terminal of a switching device, the switchingdevice compensation circuit comprising: a second threshold voltagechange detection unit configured to detect a change in threshold voltageof the switching device from a current flowing through the switchingdevice; a second control signal generating unit configured to generate asecond control signal in accordance with an output of the secondthreshold voltage change detection unit; and an offset voltage controlunit configured to control an offset voltage for the control pulse inaccordance with an output of the second control signal generating unit,wherein the second threshold voltage change detection unit includes: animpedance device coupled in series to the switching device; a detectorconfigured to detect a voltage developed across the impedance device;and a sampling unit configured to sample an output signal of thedetector and to hold and outputs a valley voltage detected from theoutput signal of the detector.
 9. The switching device compensationcircuit as claimed in claim 8, wherein the sampling unit includes asampling circuit comprising: an AC component extracting unit configuredto extract an amplitude component of an AC signal from an input signal;a first adder configured to subtract an output of the AC componentextracting unit from the input signal; an averaging unit configured toaverage an output of the first adder; a rectifying unit configured torectify the output of the AC component extracting unit; a smoothing unitconfigured to smooth an output of the rectifying unit; and a secondadder configured to subtract an output of the smoothing unit from anoutput of the averaging unit and outputs a bottom-hold value.